This invention generally relates to apparatus for printing characters and more specifically to control circuitry for such printing apparatus.
Various instruments and other data sources produce information for display as alphabetic, alphanumeric, or other characters by output devices, such as line printers. This invention is particularly applicable to a class of line printers comprising a plurality of printing actuators which are energized to perform the printing operaion. In one such line printer a printing drum in the form of a set of type wheels rotates in unison on a common shaft intermittently between successive positions. A printing hammer is actuated whenever a comparison circuit indicates that a symbol or character on the type wheel then facing the material to be printed corresponds to the character actually to be printed by that wheel.
In prior printers of this type, such as shown in U.S. Pat. No. 3,731,622, which issued May 8, 1973 and is assigned to the same assignee as the present invention, the printer includes a separate comparison circuit for each column to receive, continuously, signals representing a character to be printed in that column. Each comparison circuit energizes a printing actuator constituted by a hammer drive solenoid when the printing drum is oriented so the selected character is positioned properly for printing in the column associated with that circuit. The solenoid thereupon actuates a printing hammer to cause the character to be printed. If the same character is to be printed in two or more columns, the corresponding drive solenoids are energized simultaneously.
There are also line printers which use matrix printing techniques. Generally, these line printers include a printing head for selectively forming a plurality of dots on the printable medium. Both thermal and mechanical printing heads are used. One type of thermal printing head includes thermal elements arranged in a conventional seven-segment bar code array. The thermal elements then constitute the printing actuators. Selective energization and heating of the bar-like segments thus provides printing of selected characters and symbols. In another head the thermal elements are dot-like instead of segmented and they are arrayed in a matrix, such as the familiar 5 .times. 7 dot matrix. By selectively heating these elements, one may print a combination of dots forming any desired symbol.
A third type of thermal printing head comprises a single column of dot-like thermal printing elements. The elements are selectively energized as the tape moves past, thus printing symbols in a two-dimensional dot matrix by printing, in succession, closely spaced columns of dots.
In these diverse types of line printers, the control circuit selects certain printing actuators to be energized and then energizes all of them simultaneously. For example, in a twelve-column line printer which uses the print wheels, the minimum load on the power supply is that required to actuate one hammer while the maximum load is twelve times the single hammer load. Likewise, with a seven-element thermal printer the maximum load is seven times the load for energizing a single printing element. As it is necessary to design the power supply for a maximum load condition, the potentially large loads increase the overall cost of the power supply and the printer. It is also difficult to route the energy evenly to each printing actuator when they are all energized simultaneously. Uneven energization may produce uneven printing.
Therefore, it is an object of this invention to provide a printer which reduces the power supply requirement.
Another object of this invention is to provide a printer which provides more even printing.
Still another object of this invention is to provide a printer which is simple to design, reliable, of small size and relatively inexpensive to manufacture.